Hiding my bump. Help?

Carmel • First time mom!
Ok so I need to hide my pregnancy until at least week 13 and I'm week 7 now.
My older sister is getting married around that time and there is no wayyyyy I'm telling anyone before her big day. she's been dreaming about it for ever and I'm not going to steal all the drama from her (plus this pregnancy is probably the worst thing that's going to happen to her this year) 
I'm so scared I'll show by then? Any advice from other girls hiding their pregnancy?